1. Field of the Invention
The present invention relates generally to mobile communication systems. More particularly, the present invention relates to a system and method for estimating the power of a communication signal or set of signals forming a subbeam. The present invention is most applicable in a mobile communications system using satellites where keeping track of satellite power is critical.
2. Related Art
A satellite-based communications system is a means by which information is sent over a substantial distance. Typical satellite-based communications systems use base stations referred to as gateways, and one or more satellites to relay communications signals between the gateways and one or more user terminals. Gateways provide communication links from each user terminal to other user terminals or users of other connected communications systems, such as a public telephone switching network. The user terminals can be fixed or mobile, such as a mobile telephone, and are located anywhere they can communicate with a satellite.
A satellite transponder is the component in a satellite that receives and transmits signals from and to gateways and user terminals. A satellite transponder must be able to carry a large number of subscribers simultaneously in order to be cost effective. Various satellite access schemes such as time division multiplex access (TDMA) and code division multiplex access (CDMA) spread spectrum allow access to transponders by a large number of subscribers. Digital CDMA is preferable to other satellite access schemes as more communication signals can be carried at a lower cost and higher quality. This is due in part because CDMA systems enable use of low powered signals which minimize cross channel interference and conserve satellite power.
In a typical spread-spectrum communication system, preselected pseudorandom noise (PN) code sequences are used to modulate or xe2x80x98spreadxe2x80x99 user information signals over a predetermined spectral band prior to modulation onto a carrier for transmission as communication signals. PN spreading is a method of spread-spectrum transmission that is well known in the art.
In a typical CDMA spread-spectrum communication system, channelizing codes are used to discriminate between signals for different users within a cell or between user signals transmitted within a satellite beam, or sub-beam, on a forward link (i.e., the signal path from the base station or gateway to the user transceiver). That is, each user transceiver
In a CDMA system each customer, subscriber, or user terminal is assigned an individual, orthogonal, communications channel by using xe2x80x98coveringxe2x80x99 or xe2x80x98channelizingxe2x80x99 orthogonal codes. Walsh functions are generally used to implement the channelizing codes, with a typical length being on the order of 64 code chips for terrestrial systems and 128 code chips for satellite systems. CDMA systems combine individual code channels into a single narrowband channel so that a large number of channels are spread throughout the same waveform. As a result, multiple customers or users simultaneously share the same xe2x80x9cnarrowband channel,xe2x80x9d which is referred to interchangeably herein as a xe2x80x9cCDMA channelxe2x80x9d xe2x80x9csubbeamxe2x80x9d or a xe2x80x9ccarrierxe2x80x9d. Because multiple customers or users share the use of the same subbeam, if one or more customer or user signals are transmitted at a higher power than signals intended for other customers or users on the channel, interference may occur which may result in unacceptable performance unless the number of users on the subbeam is reduced. More importantly, such extra power reduces the power available for other user signals and, thus, overall capacity.
In a typical CDMA system, a gateway and a satellite communicate via links which are spatially divided into a number of beams, for example 16, in both a forward and a return direction, referred to as links. On the forward link, information is transmitted by a gateway generally utilizing frequency division and polarization multiplexing. In an exemplary system design, the forward link uses a C-band frequency band that is divided into 8 individual 16.5 MHz xe2x80x9cchannelsxe2x80x9d or xe2x80x9cbeamsxe2x80x9d employing right hand circular polarization (RHCP) and 8 individual 16.5 MHz xe2x80x9cchannelsxe2x80x9d or xe2x80x9cbeamsxe2x80x9d employing left hand circular polarization (LHCP). These individual 16.5 MHz channels are in turn made up of 13 xe2x80x9csubchannelsxe2x80x9d or xe2x80x9csubbeams,xe2x80x9d each of 1.23 MHz bandwidth, that are frequency division multiplexed (FDM) together to form a beam. These FDM subbeams are the narrowband channels discussed above, formed by combining a number of code channels.
For transmission to a satellite, individual FDM subbeams are frequency multiplexed together to create one wideband channel. A wideband channel has a pre-selected bandwidth designed for the specific satellite system. In the present example, a bandwidth of 160 MHz is used which comprises 104 subbeams, 13 subbeams times 8 beams. The ability of a wideband channel to carry 104 subbeams is dependent on limiting the power of each subbeam to the minimum power necessary for high quality transmission. Thus, control of the power of the subbeams is needed for high quality transmission and to ensure efficient use of power which allows the maximum number of subbeams to be carried on a wideband channel.
A system and method for controlling the gain of individual narrowband channels (subbeams) using a wideband power measurement has been developed. That system and method uses a transmit power tracking loop (TPTL) to control the power of individual narrowband channels (subbeams) by adjusting the gain applied to a transmitted signal. This system and method is disclosed in U.S. Pat. No. 6,252,915, entitled System and Method for Gain Control Of Individual Narrowband Channels Using A Wideband Power Measurement, which is assigned to the assignee of the present invention, and incorporated, in its entirety, herein by reference. Both open loop and closed loop power control are used in the TPTL. The closed loop control requires the control of the power of each individual subbeam. To control the power of each subbeam it is necessary to determine the power of each subbeam. However, difficulties arise in measuring individual subbeam power in the time frames needed to effectively control gain. In addition, performing such power estimation can be very computationally intensive, especially for control software implementations. As a result, there is a need for an alternative system and method for determining or estimating the power of individual subbeams.
Determinations of the power of individual subbeams can also be used to monitor the power consumption of a satellite receiving the subbeams. The satellite requires power to receive and relay the subbeams. The satellite is powered by batteries which store solar energy collected by the solar panels. Because the satellite only charges while exposed to the sun, the power of the satellite is limited by the exposure of the satellite to the sun.
Because of the limited energy in a satellite, it is possible that the satellite can run out of energy. Thus, in order to properly operate the satellite, it is necessary to know how much energy is being used by each transmitted subbeam. For example, in order to divide up capacity among service providers, it is necessary to know the amount of power used in transmitting on an individual subbeam basis. Also, to protect the satellite from damage by overdriving the satellite, it is necessary to know how much power is being transmitted on each subbeam.
Proper management of the satellite battery is vital to the longevity of the satellite constellation. The energy removed from the battery in order to process traffic must be replenished during the charging time when the satellite is in the sun. If too much energy is removed to process the traffic, the satellite must stay in the sun longer or must tap the batteries"" reserve power. The operating life of the battery is degraded when the battery reserve power is accessed. The quality of satellite energy estimation is in part a function of the estimation of the power of each subbeam. More specifically, power usage of a satellite can be measured by measuring the power of signals sent to the satellite. This is because a satellite transponder transmits signals at a power that is proportional to the power of the signals received by the transponder.
Thus, there is a need to estimate the power of each subbeam that is transmitted from a gateway to a satellite. These power estimates can be used to determine power consumption and can be used in control systems that adjust the power of each subbeam. More specifically there is a need to estimate the power of subbeams in order to keep track of the power consumption of and availability in_a satellite. Additionally, there is a need to estimate the power of each subbeam in order to limit the power of each subbeam. Also, there is a need to estimate the power of subbeams in order to allocate capacity among service providers and to provide billing information. Furthermore, there is a need to estimate the power of subbeams in order to avoid overdriving satellites and to avoid violating flux density limits.
The system for estimating power should consume a minimal amount of power and have a low degree of complexity so that it occupies a minimal amount of space. This is because the size of integrated circuits or chips used to implement the power estimation system and the amount of power that a chip can handle may be limited. In addition, the larger a chip, i.e, the more logic gates on the chip, the more expensive the chip is to produce. Also, the more logic gates on a chip, the more power is required to drive the gates. Because a chip can only dissipate so much power, a chip with too many logic gates may also produce too much thermal energy, causing the chip to fail. Additionally, the more power required to drive a chip, the more expensive it is to drive the chip. Therefore, reducing the number of logic gates on a chip may reduce the cost of producing and powering the chip and increase the reliability of the chip.
The invention concerns a system and method for estimating the power of a signal in a satellite communications system. The signal is compared to and separated into a plurality of ranges by a separating means. Each range is assigned a particular output value. The output values approximate the square of the input signal based on known characteristics of the input signal. A low pass filter is used to average a plurality of the output values. In one embodiment, the low pass filter comprises an infinite impulse response filter.
A feature of the present invention is that when the input signal is represented by a predetermined number of bits, the assigned output values are represented by a number of bits which is less than twice the predetermined number of bits.
Another feature of the present invention is that when the input signal is represented by a predetermined number of bits, the assigned output values are represented by a number of bits which is less than the predetermined number of bits.
Another feature of the present invention is that when the input signal is produced by an I channel or a Q channel of a Quadriphase Phase Shift Key (QPSK) modulator, the total power of a signal created by the modulator can be determined based on a ratio of the I channel power to the Q channel power.
An additional feature of the present invention is that the output of the separating means represents an instantaneous power of the input signal and an output of the filter represents an average power of the input signal.
Still another feature of the present invention is that the output of the separating means is proportional to an instantaneous power of the input signal and an output of the filter is proportional to an average power of the input signal.